Personality traits and health outcomes: an exploration into associations and potential mechanisms

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Abstract

There were two main objectives of this thesis. First, given that personality traits have
been linked to a number of diabetes risk factors and precursors such as lifestyle and the
metabolic syndrome, our aim is to explore whether personality traits are associated with type
1 and type 2 diabetes mellitus. Second, we aim to investigate several potential mechanisms
by which personality could influence diabetes, and other health outcomes such as
cardiovascular disease and mortality.
Chapter 1 provides an introductory overview of the history of personality-health
research, and discusses strengths and limitations of different methodological frameworks.
Chapters 2-4 focus on the associations between personality and diabetes. Two studies
described in Chapter 2 examine cross-sectional and longitudinal associations between
personality and type 1 and type 2 diabetes. We used a large national sample with ten years of
follow-up. We detected positive associations between openness and neuroticism and type 1
diabetes prevalence, and negative associations between neuroticism and type 2 diabetes
incidence.
In Chapter 3, we examine relationships between personality and type 2 diabetes
incidence using aggregated personality and diabetes data on a level of the U.S. counties and
states. In a six-years follow-up study, we found no evidence that mean levels of personality
traits were associated with diabetes incidence in the U.S. states.
In the following chapter we explore whether a possible mechanism by which
personality may influence diabetes is by moderating the expression of its genetic risk. The
study described in Chapter 4 looks at interactions between personality domains and facets
with polygenic risk score for type 2 diabetes in predicting glycated haemoglobin levels using
a large community-dwelling sample. This study found a negative phenotypic correlation
between openness and glycated haemoglobin levels, though this association was confounded
by cognitive ability. Moreover, genetic risk for diabetes was more strongly associated with
glycated haemoglobin levels in people with lower levels of either agreeableness or
conscientiousness.
In Chapter 5 we move away from diabetes to discuss previously reported
contradictory results regarding the effects neuroticism has on mortality. Some of the previous
studies reported higher neuroticism being associated with higher risk of mortality, whereas
some reported that higher neuroticism was associated with lower risk of death. We tested
whether the sign of the neuroticism effect was a function of the covariates included in the
models. In a national sample with ten years of follow-up we found that neuroticism was a risk
factor for death in the models that did not include objective and self-rated health variables.
However, when these variables were included, neuroticism was related to lower risk of death.
In the last empirical chapter, Chapter 6, we explore whether autonomic nervous
system activity is a biomarker for personality traits. The first study tests whether openness is
associated with measures of sympathetic and parasympathetic nervous system activation. We
find that openness was associated with sympathetic nervous system activity under baseline
but not in the stress conditions, and that it was not associated with measures of
parasympathetic activation. The second study describes a model of associations between
neuroticism and autonomic nervous system activation, while controlling for cardiovascular
disease and depression and their mutual associations. We found that neuroticism has
independent contributions to all measures of autonomic nervous system activity, and to heart
disease, even when controlling for relevant clinical variables. Thus, autonomic nervous
system activity may explain in part observed links between personality, and heart disease and
mortality.
Finally, in Chapter 7 we summarize the findings presented in the five empirical
chapters, discuss the limitations of the current method, and offer suggestions for future
research in the field.